In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Watch those instruments!
Date: Apr 03, 9:34 am
Poster: Richard Hull
On Apr 03, 9:34 am, Richard Hull wrote:
I was running fusor III this weekend for some more doubting thomases and can always get to 1 mrem of neutrons quickly. I use a Bonner Sphere neutron counter for the plebian audiences as it doesn't give any audible indication until the fusor starts actually making neuts. One millirem, at the distance I have it placed from the fusor, represents about 55,000 n/sec @ 30-40 cpm. It makes for a great eye opening demo.
After they had departed I was extending the system by raising the voltage and jockeying the pressure by alternately opening the pump valves by minute fractional nudgings and the D2 needle micrometer.
the counts slowly climbed to 60-100cpm or ~2mrem. This is the 10e5 neutrons/ second range which is kissing the ceiling of what I can do. I am always trying to push the envelope and continued fiddling. Suddenly, the count started climbing and climbing until I was at 4.5 mrem! I was ecstatic! As I worked the valving the count continued to climb even as I was reducing pressure and current!???? At 8 mrem I had had enough of this game and shut off the fusor power and gas flow.
The counter stayed at 8mrem clicking 5 or 6 times per second. I have never heard this! As a quick check, I quickly grabed the sphere and PNR-4 Eberline and raced across the lab to a bench. There the unit still read 8 mrem! It was the instrument!
After about 1/2 hour of electronic diagnostics, I found out it was not a problem with the electronics at all, but a leaky and or damp HV teflon surface inside the connector between the sphere and PNR-4. Alcohol on a Q tip and some judicious scrubbing and hot air blower drying restored the instrument to its normal, virtually zero background count level.
I went back to the fusor and another run showed that the 55,000 neutron level at 1mrem was easy to hit and that 2 mrem was near the peak. Leaving the system going for 1 hour had the meter responding as advertised.
I have said it before and will continue to say it... neutron counting is a real art. Others I know in the nuclear health physics sciences both at NRL and LLNL agree. Neutron counting is tough and the instruments are often flakey and tests of the instrument before and after crtical measurements are a must. BF3 systems are particluarly subject to cable problems, moisture and dirt both inside the insturments and at any interconnect point.
The reason for this is that the normal BF3 tube DC applied voltage is between 1500 and 2500 volts while on the same line, the signal is rarely over 1 milivolt. Reading this pulse on the HV DC line is electronically quite easy until you throw the thing out into the real world. The tiniest imperceptible corona will yield "Tritchel" pulses in the 100,000/sec area of a volt! (swamping the counter) This is easy to detect. Flexing a cable can often vary the capacitance enough to give spurious counts. Some of the spurious counts are so subtle that to trace the cause down can be maddening. I have heard many horror stories around the ole neutron campfire.
If you opt for a BF3 counter, you must set it up in situ. You must run it for a period of 10 minutes prior to the critical test and it should show virtually no counts (maybe .5 cpm depending on instrument and moderation). The measurement is made and data taken. The unit is left on and tested for 10 minutes following the run. If before and after are similar, you can rely on your data.
Tricky stuff and requires a calm head. Some folks attribute the early positive, but false, neutron results of cold fusion in 1989 and 1990 to electrochemists playing with neutron counters for the first time. Harwell's neutron lab spent months on the issue and by late 1990 found zero neutrons in cold fusion cells.
Without positionng myself on that issue, it seems that if fusion is occuring in current work on that subject, the process does not release the neutrons from the lattice. Secondly, their exchange is in a fashion yet to be understood within the bounds of normal high energy physics.
- Re: Watch those instruments! - IJV
- Re: Watch those instruments! - IJV Apr 04, 9:03 pm
- Re: Watch those instruments! - IJV Apr 04, 9:00 pm